1. Field of the Invention
This invention relates to industrial process control systems used with processes which are spread out geographically over relatively large areas, yet are to be controlled from a central operating station. More particularly, this invention relates to improved means for handling the transfer of data and control instructions between the central station and the geographically remote regions of the process, including data from sensing elements and instructions for operating elements such as process valves.
2. Description of the Prior Art
A considerable number of process control systems have been proposed and described in the art, and many have gone into commercial use. In more recent years, digital computers have been extensively used with process control systems to monitor functioning of the process and to direct the overall functioning of the control system. Such computer-directed systems have generally been classified as (1) supervisory control systems, wherein the computer develops set-point signals for individual controllers associated with each process valve, or (2) direct digital control systems, wherein the computer develops valve-control signals to be sent directly to valve-positioning equipment for each valve. Selected prior art disclosures of typical systems may be found in U.S. Pat. No. 3,582,621 (Lawler), U.S. Pat. No. 3,761,889 (Halley et al), and U.S. Pat. No. 3,971,000 (Cromwell).
The earlier computer-control systems generally used conventional solid wire conductors for transferring data, in the form of DC signals, between the central station and the system elements in the field. Thus, electrical conductors in heavy shields were led from the sensing elements, valves, etc., to junction boxes, and from there to central or other processors. For relatively large systems, the cost of such conventional wired data transfer networks has become very great, because they are expensive to install and involve high costs of planning, layout, and documentation. Improvements have been effected by transmitting the data part of the way in digital format through cables which serve a number of process elements, as disclosed in U.S. Pat. No. 3,760,374 (Nabi). But nevertheless it is clear that there is an urgent need for a fresh solution to the problem of transferring process information and control signals between a central location and outlying regions of a geographically-dispersed process.
In a process control system, the transfer of measurement and control signal data must meet very stringent requirements. Paramount is the need for reliability, since the loss of proper signal information can lead to severe injury and costly damage to a process. It also is necessary to provide for privacy and freedom from external interference. The wired signal transfer arrangements used heretofore have been excellent in all these respects, but as noted above, this goal has been achieved only at very great economic cost. Thus it is desired to provide for data transfer at significantly reduced cost, while still maintaining the highest of reliability and other important quality standards.
It has been known that data transfer in a process control system could be effected at relatively low cost by the use of modulated high-frequency signals, particularly by radio carrier links. However, no appreciable use of such techniques has heretofore been made in process control systems, primarily because the relatively low cost types of transmission equipment were viewed as being insufficiently reliable or free from external interference. For example, it is clear that the commonly-used amplitude and frequency modulation systems are subject to considerable interference from the electrical noise sources typically encountered around an industrial process.